Cobalt and rare earth metals are known as valuable metals and used for various applications in industry. Cobalt is used for positive electrode materials for secondary batteries, and, furthermore, superalloys (high strength heat-resistant alloys) used for e.g. jet engines for aircraft, and the like. Rare earth metals are used for fluorescent materials, negative electrode materials for nickel-hydrogen batteries, additives for magnets installed in motors, abrasives for glass substrates used for liquid crystal display panels and hard disk drives, and the like.
In recent years, energy savings have been strongly promoted, and in the automobile industry, conventional gasoline-engined cars are being rapidly replaced by hybrid cars and electric cars equipped with secondary batteries using cobalt and rare earth metals. In lighting equipment, conventional fluorescent lamps are being rapidly replaced by efficient three band fluorescent lamps using rare earth metals such as lanthan, cerium, yttrium, terbium and europium. The above cobalt and rare earth metals are scarce resources, and most of them depend on imports.
Yttrium and europium have been used for fluorescent substances in cathode ray tube television sets in analog broadcasting; however, in recent years, large numbers of cathode ray tubes have been put out of use because of the transition to liquid crystal television sets. Products which have rapidly spread, such as secondary batteries and three band fluorescent lamps, can be also easily expected to cause a large amount of waste in the future as used products. Thus, cobalt and rare earth metals, scarce resources, are treated as waste without recycling of the used products, which is not preferred in terms of resource savings and resource security. Nowadays, the establishment of a method for effectively retrieving valuable metals such as cobalt and rare earth metals from such used products is strongly demanded. Retrieval of cobalt from secondary batteries
The above secondary batteries, incidentally, include nickel-hydrogen batteries, lithium ion batteries and the like, and in addition to cobalt, a rare earth metal, manganese is used for positive electrode materials thereof. In positive electrode materials in lithium ion batteries, the ratio of low cost manganese tends to be increased in place of high cost cobalt. The retrieval of valuable metals from used batteries has been attempted recently, and as one of the retrieval methods, there is a dry method in which used batteries are thrown into a furnace and melted, and metals and slag are separated to retrieve the metals. In this method, however, manganese moves to slag, and thus the metal that can be retrieved is only cobalt.
Further, a wet method is known, in which used batteries are dissolved in an acid and metals are retrieved using a separation method such as a precipitation method, a solvent extraction method or an electrowinning method. In the precipitation method, for example, a method in which the pH of a solution comprising cobalt and manganese is adjusted and a sulphurizing agent is added thereto to obtain a cobalt sulphide precipitate, and a method in which an oxidizing agent is added thereto to obtain a manganese oxide precipitate are known (see Patent Document 1). The methods, however, have problems such as the occurrence of coprecipitation, and it is difficult to completely separate cobalt and manganese.
It is known that when cobalt is retrieved as a metal by the electrowinning method, in a system in which a high concentration of manganese exists, manganese oxides are precipitated on the surface of an anode and deterioration of the anode is promoted. In addition, a peculiar colored fine manganese oxide is suspended in an electrolytic solution, and thus, for example, clogging of the filter cloth used for electrowinning and contamination of cobalt metal by the manganese oxide occur. Therefore, stable operations are difficult.
When cobalt is retrieved using the solvent extraction method, an acid extraction agent is widely used. As described above, however, nowadays manganese is used in large amounts for positive electrode materials in lithium ion batteries, and thus a high concentration of manganese exists in the solution in the batteries. Efficient extraction agents which selectively and efficiently extract cobalt from such a system do not exist in the existing circumstances.
In addition to recycling used batteries, in cobalt smelting which is currently carried out to produce cobalt, a raw material is a nickel ore such as a nickel oxide ore. The percentage of manganese is however higher than that of cobalt in the nickel oxide ore, and the existing percentage thereof is about 5 to 10 times that of cobalt. When cobalt is smelted, separation from manganese is a major problem.    Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2000-234130    Non-Patent Document 1: K. Shimojo, H. Naganawa, J. Nora, F. Kubota and M. Coto; Extraction behavior and separation of lanthanides with a diglycol amic acid derivative and a nitrogen-donor ligand; Anal. Sci., 23, 1427-30, 2007 December.